PORTLAND, Ore. - Usually by now the Columbia River's spring chinook salmon are heading upstream over fish ladders in the tens of thousands to spawn. But not this year.

"It's a never-before-seen scarcity," said Charles Hudson of the Columbia River Inter-Tribal Fish Commission. "We're way behind, even compared to the historically low years of 1994-1995."

It's this bad: For centuries the treaty Indian tribes on the river have caught salmon for the ceremonial First Foods celebration marking the return of the fish. This year they had to get their salmon somewhere else.

Fish biologists had predicted a spring run of about 229,000 chinooks at Bonneville Dam, about 140 miles upstream from the Pacific. But as of Tuesday, near the usual midpoint of the spring run, only about 200 had been counted there.

The chinooks enter the Columbia River from the Pacific at this time of year to return to the streams where they were hatched.

Scientists say they don't have an explanation.

"Nobody knows why, certainly not any of the scientists here I've talked to," said Brian Gorman of the Pacific Marine Fisheries Service in Seattle. "It's a mystery. Nobody has a clear idea."

Gorman said the run is not only late, "it is mysteriously late."

Some fish managers wonder whether low water levels as a result of a dry winter — combined with murky water caused by recent rains — are keeping chinook from swimming up the Columbia.

"Spring chinook are pretty finicky when conditions are abnormal," said Guy Norman, regional director for the Washington state Department of Fish and Wildlife. "April and early May are the most significant times for spring chinook movement over the (Bonneville) dam. We're hoping for good things to come."

Fish swimming upstream on the Columbia are tallied at the Bonneville Dam, where they go up fish ladders, which resemble stairs, and swim past a large window.

Their numbers are a factor in setting fishing seasons for sport, tribal and commercial fishermen.

Hudson said he's optimistic "there are fish out there gathering at the mouth of the river waiting for some biological trigger to send them up."

The economic impact of the small chinook return is not clear.

"Historically the midpoint of the run is April 26 at Bonneville Dam," Hudson said. "We will have to wait and see where we are then, when we may have a clearer picture."

Curtis Melcher, marine salmon fisheries manager for the Oregon Department of Fish and Wildlfe, said weekly meetings are held to look at the size of the run and the size of the catch, and regulators are not yet ready to recommend trimming the season.

He said sport fishermen are catching some chinooks but not as many as usual. He said many of those caught were bound for the Willamette River and other tributaries below the dam.

Each year juvenile salmon swim downstream through an obstacle course of hydroelectric dams to the Pacific Ocean. In two or three years, through a biological instinct that is not well understood, they return upriver to the stream where they were hatched. There, they spawn and die.

Most of this year's spring run went to sea in 2002 or 2003, said Norman, adding that there were no conditions in those years that would readily explain the dearth of fish this spring.

Hudson said the fish are back in near their usual numbers in the Willamette River, which joins the Columbia well below Bonneville Dam, the first dam the returning fish encounter on their return.

"With an impact of this kind you're usually talking about hydroelectric operations as a likely cause," Hudson contended.

Bonneville is required to release a certain amount of water past dams to help fish if the water flow is low to keep young salmon out of hydroelectric turbines. The turbines kill about 10 percent of the fish that go through them.